1. Field of the Invention
The present invention relates to a flow controlling apparatus for a power steering system which constantly supplies operational oil from an oil pump to a hydraulic device to assist a steering wheel of a vehicle by hydraulic power.
2. Description of the Related Art
It is well known for a flow controlling apparatus for a power steering system to output a predetermined volume of the discharged operational oil to a hydraulic device in accordance with a vehicle speed or a steering angle of a steering wheel, for example as shown in Japanese laid-open patent publication No. 2001-163233. In this related art, the discharged fluid is controlled by a variable valve equipped in an outlet of the flow controlling apparatus by energizing a solenoid. The solenoid is controlled by an electric signal relating to the vehicle speed or the steering angle through control means such as a micro computer. The solenoid is energized in accordance with the electric signal to operate the variable valve in order to control the amount of fluid to the hydraulic device of the power steering system.
In the well known power steering system, since the solenoid is energized in accordance with the electric signal relating to the vehicle speed or the steering angle of the steering wheel to operate the variable valve in order to control the amount of fluid supplied to the hydraulic device of the power steering system, it is best for energy saving that the amount of discharged fluid is zero where the steering is judged to be not operated by judging the steering angle. However, where the amount of discharged fluid is zero, seizure of the hydraulic device or unsteady driving based on a disturbance such a kick-back and so on in a straight run can occur. Further, it requires time during rapid steering for the discharged fluid volume to increase to a suitable amount, causing delayed response of power assistance by the hydraulic device. Because of this reason, the well known flow controlling apparatus keeps a minimum flow of fluid discharged to the hydraulic device of the power steering as standby fluid when waiting to assist the steering wheel. Therefore, it is easy to reach the predetermined amount of fluid from the standby fluid amount even for rapid steering. The well known electrically controlled flow controlling device provides only a single way of transition from standby amount of fluid Qs to a suitable amount of fluid (Qa) needed for assisting the hydraulic device, that is to say the time constant relating to an increase from Qs to Qa is only a single value, as shown in FIG. 1. Therefore, where the time constant is set for rapid steering in the control means, the assisting force suddenly and unexpectedly rises at slow steering, thereby making the driver feel uncomfortable.